It is customary when making a flight plan, or when making an approach for a landing, that the flight or portions thereof be made at predetermined altitudes customarily under the supervision of air traffic control. For example, it may be determined according to certain flight plans that the aircraft should assume an altitude of, say, 5,000 feet and maintain this throughout the given course. Or in preparation for landing, because of other traffic, air traffic control may decide that the aircraft should approach from a given direction or along a given omni radial and maintain a certain altitude during interception of the new course. The pilot, of course, is at the same time involved in other activities concerning the flight of the aircraft including being on the alert for other traffic. Accordingly, the pilot may inadvertently allow the aircraft to wander from the given altitude and thereby pose a hazard to other aircraft in the vicinity or to his own aircraft due to local obstacles.
It has been conventional in the past, particularly in the handling of private planes, to rely solely upon the attentiveness of the pilot to monitor altitude and comply with air traffic control requirements in this regard.